In many types of electronic circuits, it is necessary to generate an internal clock signal. A simple circuit for generating such a signal is a conventional ring oscillator circuit, such as shown in FIG. 1. Such a circuit 10 includes a plurality of series-coupled inverters 12a, 12b, and a cross-coupled feedback path 14 connecting the output of the last inverter 12b to the input of the first inverter 12a. In order for such a circuit to oscillate, the overall gain in the loop must be greater than one. In addition, the overall phase shift must be 180 degrees, and thus at least two stages arc required to introduce the two poles required for this phase shift. The frequency of oscillation of a conventional ring oscillator is related to the delay between each of the stages, which delay is a result of the type of inverter circuit used as well as the magnitude of parasitic capacitances, Cp, which are present in the circuit. If each stage introduces a delay of .tau., the loop delay, corresponding to the time to complete one-half a cycle, is 2.tau.. Thus, the nominal frequency of operation for a two-stage circuit, such as in FIG. 1, is 1/4.tau..
Although easy to construct, ring oscillators arc not particularly effective at high frequencies relative to the limits of technology at issue. For example, in a particular technology, a latch can be clocked at 40 GHz. In that technology, a two-stage ring oscillator typically cannot generate an output signal which has a frequency greater than 20 GHz due to the switching delay of each stage. Delay of inverters which are used in conventional ring oscillators can be reduced by decreasing their load resistance. Although this increases the operating frequency of the ring oscillator, it also reduces the small signal gain at the operating frequency. Thus, such an oscillator generally has low power at the output and is therefore subject to unacceptable levels of noise.
An alternative oscillator, suitable for high frequency applications, makes use of LC resonator circuits. A wide variety of LC oscillators are known to those of skill in the art. A drawback to this class of circuits is that they can take up a large amount of area on an integrated circuit. A further drawback is that they have a very limited range of tunability and thus require high precision during fabrication, increasing overall cost.
Accordingly, it would be advantageous to provide a modified ring oscillator configuration which has reduced loop delay and a higher gain at the desired frequency of oscillation than conventional ring oscillator circuits to thereby provide increased oscillation frequencies.